Meeting the demand for isotopic carbon dioxide and methane gas reference materials for underpinning global observations

To support governments verifying emissions and demonstrating national reduction targets, it is necessary to discriminate between the natural and various man-made sources of greenhouse gases. This requires accurate measurements of baseline concentrations and contributions resulting from emission events. Separating manmade emissions from measured carbon dioxide and methane amount fractions is challenging and requires information on the isotopic composition, especially if man-made negative emissions start to play a role. Currently there is no infrastructure to meet the demand for carbon dioxide and methane gas reference materials with the required uncertainties to underpin global observations, compromising the comparability of measurement data. It is therefore necessary to address the existing traceability gap in the measurement of isotopes of carbon dioxide and methane by developing gas reference materials, calibration methods and dissemination mechanisms, which are traceable to existing scales (e.g. VPDB - Vienna Pee Dee Belemnite - and VSMOW/SLAP - Vienna Standard Mean Ocean Water/ Standard Light Antarctic Precipitation) and the SI. Additionally, metrology is also required to ensure advances in optical spectroscopy result in field deployable techniques that meet uncertainty requirements. Metrology is required to address the following challenges: • The production of carbon dioxide gas reference materials from carbonates for underpinning isotope ratio is currently limited and prohibitively expensive. Independent capabilities for the whole traceability chain and an improved understanding of the influence of parameters in gravimetric preparation on isotopic fractionation is essential. Gas reference materials of carbon dioxide and methane traceable to the VPDB VSMOW/SLAP scales for δ13C, δ18O and δ2H respectively, are needed to meet the uncertainty and high-volume requirements to underpin global measurements created by the mass adoption of commercial spectroscopy. There is also no Central Calibration Laboratory (CCL) at the WMO-GAW level for methane to ensure compatibility of global observations. • No absolute isotope ratio measurements traceable to the SI have been achieved with the desired uncertainty due to insufficient methods and instrumentation. Isotope ratio remains a traceability exception under the CIPM-MRA and SI traceable methods are required for absolute isotope ratio measurements of carbon dioxide and methane. • Introduction of relatively low-cost spectroscopic techniques has revolutionised the measurement of key greenhouse gas components in air by enabling continuous in-situ field measurements for quantifying sources and sinks at local, regional, and global levels. In tandem, existing validation routines, recommendations and traceability chains for field-deployable spectroscopic techniques that meet the precision specifications of IRMS are required.